Lecture 30

Regular expressions

MCS 260 Fall 2021
David Dumas

Reminders

Loose end: Recursion pros and cons

Often can solve a problem with recursion or with loops (an iterative solution). Why use recursion?

raw strings

Recall that backslash \ in a string starts an escape sequence in Python.

You can disable escape sequences by putting the letter r immediately before the quotation mark(s). This is known as a raw string. In a raw string, a single \ represents the \ character.

However, raw strings cannot end with a single \

    >>> print("C:\\Users\\ddumas\n(home)")
    C:\Users\ddumas
    (home)
    >>> print(r"C:\\Users\\ddumas\n(home)")
    C:\\Users\\ddumas\n(home)
    >>> print(r"C:\Users\ddumas")
    C:\Users\ddumas
    >>> 

Minimal example

Simplest usage: Find and replace a substring.

import re
s = "Avocado is usually considered a vegetable."
print(re.sub("vegetable","fruit",s))
    

The first argument of re.sub is a pattern.

Unless it contains characters with special meaning in a regex pattern, the pattern just matches substrings equal to the pattern.

• "vegetable" matches the string "vegetable"
• "foo" matches the string "foo"

special characters in patterns

• . — matches any character except newline
• \s — matches any whitespace character
• \d — matches a decimal digit
• \w — matches a "word character" (a-z, A-Z, 0-9, _)

special characters for repetition

• + — previous item must repeat 1 or more times
• * — previous item must repeat 0 or more times
• ? — previous item must repeat 0 or 1 times
• {n} — previous item must appear n times

Example problem

Replace any price in whole dollars (written like $2 or $1999) with the string -PRICE-.

Note: $ is a special character. To match a dollar sign, put \$ in the pattern.

Searching without replacing

• re.match(pattern,string) — does string begin with a match to pattern? Return a match object or None.
• re.search(pattern,string) — does string contain a match to the pattern? Return a match object or None.
• re.finditer(pattern,string) — return an iterable yielding all the non-overlapping matches as match objects.

Match objects

Most regex functions return match objects that contain info about a part of the string matching the expression.

A match object has a method .group() that returns the full text of the match.

.start() and .end() return the indices where the match begins and ends in the string.

Parentheses

A part of a pattern in parentheses is a group. A group is treated as a unit for operators like +,*,?.

e.g. pattern (ha)+ means one or more repetitions of ha.

It matches ha or haha or hahaha but does not match Haha or h or hah.

In contrast, ha+ means the letter h followed by one or more repetitions of a, e.g. haaaaaaa

Retrieving groups

Matched groups are available as .group(1), .group(2), etc., with the 1-based number referring to the order of left parentheses in the pattern.

Group 0 always refers to the entire pattern.

e.g. pattern   My name is (\w+).   will capture the name (not containing spaces!) in group 1.

Example problem

Find all of the phone numbers in a string that are written in the format 319-555-1012, and split each one into area code (e.g. 319), exchange (e.g. 555), and line number (e.g. 1012).